A thick-film high frequency signal circuit has been widely used for a tuner and a demodulator in radio receivers and/or television receivers. The thick-film high frequency signal circuit is housed in a shielding case in order to prevent an electromagnetic disturbance output from the circuit or input to the circuit. Generally, signal input terminals and/or signal output terminals for the thick-film high frequency signal circuit are provided on the shielding case via feedthrough capacitors. As is well known, the feedthrough capacitor operates to reduce noises entering into the circuit via terminals.
Referring now to FIGS. 1 and 2, a feedthrough capacitor 20 used in a conventional thick-film high frequency signal circuit apparatus will be described in brief. FIG. 1 shows a plan view and a sectional elevation of a dielectric body 21 for the feedthrough capacitor 20. FIG. 2 shows a manufacturing process of the feedthrough capacitor 20.
As shown in FIGS. 1(a) and 1(b), the dielectric body 21 made of ceramic is defined a through hole 22 at the center of the ceramic dielectric 21. A pair of electrodes 23 and 24 are attached on both sides of the ceramic dielectric 21 in orthogonal to the hole 22, as shown in FIG. 2(a). The electrodes 23 and 24 are defined holes with a diameter larger than the diameter of the through hole 22, respectively. A tin-coat conductive rod 25 is fit through the hole 21, as shown in FIG. 2(b). One end 25a of the conductive rod 25 is soldered to the electrode 23 with solder 26. Other end 25i b of the conductive rod 25 is extended through the electrode 24 without any conduction to the electrode 24. Thus, the feedthrough capacitor 20 is formed.
The feedthrough capacitor 20 is mounted on an outside wall of a shielding case 27 for housing a thick-film circuit 28, as shown in FIG. 3. FIG. 3 is a sectional elevation showing a conventional thick-film high frequency signal circuit apparatus 29.
In FIG. 3, the electrode 24 of the feedthrough capacitor 20 is soldered or bonded to the shielding case 27 by conductive bond 30, so that the conductive rod 25 penetrates both opposite walls of the shielding case 27 through holes 31 and 32 defined in the walls of the shielding case 27.
The conventional thick-film high frequency signal circuit apparatus 29, as shown in FIG. 3, comprises the feedthrough capacitor 20, the shielding case 27 and the thick-film circuit 28 housed inside the shielding case 27. The thick-film circuit 28 is constituted on a substrate 33. Some conductive layers 34a, 34b, 34c, . . . and some thick-film electrical components, e.g., a thick-film resistor 35 are formed on the substrate 33 by a conventional thick-film technique. The substrate 33 defines a through hole 36 at a position facing the feedthrough capacitor 20. Thus, the conductive rod 25 passes therethrough. The middle part of the conductive rod 25 is soldered to one of the conductive layers, i.e., the conductive layer 34a around the hole 36 on the substrate 33 by solder 37. Thus, the conductive rod 25 of the feedthrough capacitor 20 is coupled to a signal line in the thick-film circuit 28 through the conductive layer 34a. Furthermore, the head of the other end 25b of the conductive rod 25 protrudes from the opposite wall of the shielding case 27. The protruding end of the conductive rod 25 constitutes a pin terminal 38 for receiving input signals to the thick-film high frequency signal circuit apparatus 29 or outputting signals therefrom.
The conventional thick-film high frequency signal circuit apparatus, as shown in FIG. 3, has some drawbacks as follows.
In the thick-film high frequency signal circuit apparatus 29, the feedthrough capacitor 20 is mounted outside the shielding case 27. Thus, the thick-film high frequency signal circuit apparatus 29 becomes large in size. Feedthrough capacitor 20 gives a poor appearance as it projects out of the shielding case 27. The feedthrough capacitor 20 also has an insufficient moisture-resistance due to the feedthrough capacitor 20 being mounted outside the shielding case 27.
A manufacturing process of the thick-film high frequency signal circuit apparatus 29 is very complicated. That is, the feedthrough capacitor 20 must be soldered to both the shielding case 27 and the conductive layer 24c of the thick-film circuit 28 after the thick-film circuit 28 is housed in the shielding case 27.
Further, an electromagnetic shielding of the thick-film high frequency signal circuit apparatus 29 is insufficient. Now the electromagnetic shielding of the conventional thick-film high frequency signal circuit apparatus 29 will be explained in reference to FIG. 4. FIG. 4 is an equivalent circuit diagram of the thick-film high frequency signal circuit apparatus 29, in which a plurality of the feedthrough capacitor 20 are provided for receiving signals therein or outputting signals therefrom.
As shown in FIG. 4, the feedthrough capacitor 20 is coupled to the opposite side of the pin terminal 38 in respect to the signal line, i.e., the conductive layer 34c. Such a circuit configuration is not desirable when viewed from a noise reducing object against noises, such as the electromagnetic disturbance, incoming or outgoing through the pin terminal 38.